a) Field of the Invention
The present invention relates to a method for producing gaseous hydrogen by chemical reaction of a metal or a metal hydride that is or has been subjected to intense mechanical deformations.
The invention also relates to an apparatus for producing gaseous hydrogen, which is specially desired to carry out the above method.
The invention further relates to a method for producing gaseous hydrogen by chemical reaction of a nanocrystalline metal hydride.
In the following description and appended claims, the term xe2x80x9cnanocrystallinexe2x80x9d is used to identify products whose particles have nanocrystalline grains with an average size of 3 to 300 nm. The term  less than  less than chemical hydride greater than  greater than  as used hereinafter means a metal hydride that can be used in a chemical reaction to produce hydrogen or other secondary products.
b) Brief Description of the Prior Art
It is known that CaH2, NaH and LiH can be used as chemical reactants together with water to produce hydroxides and hydrogen by a reaction that is called  less than  less than hydrolysis greater than  greater than :
CaH2+2H2Oxe2x86x92Ca(OH)2+2H2xe2x80x83xe2x80x831)
NaH+H2Oxe2x86x92NaOH+H2xe2x80x83xe2x80x832)
LiH+H2Oxe2x86x92LiOH+H2xe2x80x83xe2x80x833)
It is also known that hydrogen can be released by reacting pure metals with water, as in the following reaction:
Mg+2H2Oxe2x86x92Mg(OH)2+H2xe2x80x83xe2x80x834)
The above reactions and industrial processes using them to produce gaseous hydrogen have been known for a long time.
For instance, U.S. Pat. No. 3,787,186 (1974) entitled xe2x80x9ccalcium hydride gas generator greater than  greater than  discloses a gas generator in which CaH2 is reacted with water to generate hydrogen gas to activate a gas operated pump.
U.S. Pat. No. 5,372,617 (1994) entitled  less than  less than Hydrogen generation by hydrolysis of hydrides for undersea vehicle fuel cell energy systems greater than  greater than  discloses a hydrogen generator for hydrolyzing hydrides to provide hydrogen on demand to a fuel cell, the water for the reaction being provided as the by-product of the fuel cell.
U.S. Pat. No. 5,833,934 (1998) entitled  less than  less than Demand responsive hydrogen generator based on hydride water reaction greater than  greater than  discloses a novel generator configuration for reacting an alkali or alkali-earth metal hydride with water to generate hydrogen.
U.S. Pat. No. 5,593,640 and U.S. Pat. No. 5,702,491 (1997) entitled  less than  less than Portable hydrogen generator greater than  greater than  disclose a hydrogen generator and a method for generating hydrogen by hydrolysis. This method requires that the chemical hydride be heated prior to hydrolyzing.
In two papers entitled a Hydrogen transmission/storage with a metal hydride/organic slurryxe2x80x9d and a  less than  less than Hydrogen for a PEM fuel cell vehicle using a chemical-hydride slurry greater than  greater than  published in the proceedings of the 1999 US DOE Hydrogen Program Review, Ronald W. Breault et al. disclose a process wherein a chemical hydride is mixed with an organic compound such as a mineral oil in a 50/50 mixture to stabilize the product and the so prepared chemical hydride/organic slurry is reacted with water to release hydrogen.
The main problems with these existing processes to produce hydrogen, are that the hydrolysis reaction is often incomplete or proceeds either at an insufficient rate or, in other cases, at a too high or uncontrollable rate (explosive reaction like in the case of LiH). With conventional metal hydrides, the hydroxide film which is formed on the surface of the material during hydrolysis may passivate this surface and stop the reaction. The following are examples of problems described by some authors:
 less than  less than sodium hydride has a base-limited hydrolysis reaction. When the pH of the system reaches approximately 13.6 the hydrolysis reaction stalls greater than  greater than ;
 less than  less than the calcium hydride was observed to be considerably slower than the sodium hydride and the lithium hydride greater than  greater than ;
 less than  less than one of the essential considerations of the metal hydride is its hydrogen generation efficiency, which includes reaction chemistry between metal hydride and water to complete hydrolysis reactions in a safe and controlled manner greater than  greater than ;
 less than  less than calcium hydride (CaH2) reacts with water to form the hydroxide Ca(OH)2 at room temperature. Any hydroxide formed below 400xc2x0 C. decomposes endothermically above 580xc2x0 C., liberating water. If there is any unreacted hydride present when the water is released, it will react instantaneously producing H2 and CaO. The hydride itself decomposes thermally only above 600xc2x0 C. greater than  greater than  (this means that if the temperature increases too much due to the exothermic reaction between CaH2 and water, a self-sustain or explosive reaction can occur); and, finally,
 less than  less than regeneration of the end products is potentially problematic greater than  greater than .
In all the above mentioned patents and articles, MgH2 has never been used in practice in spite of its low cost, probably because the reaction of MgH2 with H2O proceeds at a rate too low for practical applications and is incomplete because of the formation of passivating Mg(OH)2 layers.
On the other hand, U.S. Pat. No. 5,882,623 (1999) naming one of the present coinventors, Mr. SCHULZ, also as coinventor, discloses inter alia a method for chemically inducing hydrogen desorption from a metal hydride. In accordance with this method, a powder of a Mg-based hydride is mixed with a small amount of a powder capable of reacting with water such as LiAlH4 (see example 2). Addition of an appropriate amount of water causes a rapid and exothermic reaction (LiAlH4+4H2Oxe2x86x92Li(OH)+Al(OH)3+4H2) which releases a large amount of heat and causes desorption of hydrogen from the Mg-based hydride.
The object of the present invention is to provide a method for producing gaseous hydrogen by chemical reaction of a metal or a metal hydride, which method is an improvement to the technologies disclosed in the above mentioned patent U.S. Pat. No. 5,882,623 and solves the above listed problems.
The present invention is based on a discovery made by the present inventors that when instead of using conventional metal hydrides (Mg-based or others), use is made of a metal or metal hydride that is or has been subjected to intensive mechanical deformations, such as a metastable nanocrystalline metal hydride, the chemical reaction (especially hydrolysis) will then take place much more readily, at a much higher rate and, most of the time, up to completion (100% conversion).
This is a very important discovery for practical applications. Indeed, because of the particular microstructure and the very large number of grain boundaries and crystalline defects, nanocrystalline metal or metal hydrides especially those made by high energy ball milling, are much more reactive than conventional metal hydride. Thus, the chemical reactions (hydrolysis is a particular case) take place much more rapidly and up to completion.
Thus, the present invention provides an improved method for producing gaseous hydrogen by subjecting a metal or a metal hydride to a chemical reaction, wherein the metal or metal hydride subjected to the chemical reaction is nanocrystalline.
The invention also provides an improved method for producing gaseous hydrogen by subjecting a metal or metal hydride to a chemical reaction, wherein the metal or metal hydride is subjected before or during the reaction to intense mechanical deformations to activate said reaction.
The invention further provides an apparatus for producing gaseous hydrogen, which comprises a reactor in which a metal or metal hydride is subjected to a chemical reaction. In accordance with the invention, this apparatus also comprises means within the reactor for subjecting the metal or metal hydride to intense mechanical deformations in order to activate the chemical reaction.